Device and method for suppressing harmonic distortions at the output of variable frequency drive

ABSTRACT

A device for suppressing harmonic distortions at the output of variable frequency drive (VFD) comprising at least one passive harmonic filter (PHF) which has a set of storage elements and a set of inductive elements. Wherein said inductive elements are mounted on a magnetic core. Said PHF comprising at least two lines connected in parallel (parallel lines) with similar sets of elements. 
     Described technical solution expanding the possibility of operating the VFD/VSD with different frequencies of the supply voltage, and also allows to maintain the THD/TDD values within 5% regardless of active power fluctuations.

FIELD OF THE INVENTION

The present invention relates to electrical power distribution systems and more particularly to filters for suppressing harmonics currents distortions in such systems.

BACKGROUND OF THE INVENTION Description of the Related Art

It is well known to those skilled in the art that non-linear load is causing harmonics currents distortion of the supply voltage due to the reason of highest current harmonics, in three-phase circuits, as a rule, the 5th, 7th, 11th, 13th, etc.

The higher current harmonics of power supply systems distort the voltage shape on the buses of the supply transformers, increased the energy loss in supply networks, caused premature aging of the insulation of motors and cables, thereby reduced the reliability and durability of electrical equipment. That is why in many countries state standards have been introduced, such standards regulate the acceptable levels of harmonic voltage distortions (THD—total harmonic distortion) caused by currents of supply systems and another loads.

Electrical distribution systems, for example in an oil field or industrial facility, supply electricity to non-linear loads, such as adjustable speed motor drives of submersible electric motors or other similar loads. Such loads produce various levels of current at harmonic frequencies in the power distribution system with the harmonic spectrum depending upon the nature of the load.

It is not a secret that higher levels of Total Harmonic Distortion (THD)/Total Demand Distortion (TDD) can drastically affect the proper performance of the Artificial Lift electrical equipment and its life span in the Oil Field. The good examples of such are ESPs and cables. Moreover, the harmonic impurity of the sideline affects surface equipment too within an electric power distribution network.

Some examples of downhole and underground systems described in patent documents: UA115401, UA118287, UA118520, RU2615775, as well as patent applications WO/2019/108160, US20170284177A1 and US20190069441A1.

With an increasing number of oil wells drilled and operated every year, the isolation for poor quality distribution systems and mitigation of harmonics becomes more and more important. Thus, as per IEEE 519, for Electric Power Systems it is suggested to keep THD/TDD levels as low as possible and this becomes a shared responsibility of system owners, operators, and users.

United States patent application (No. US 2006/0197385 A1, H02J 1/02, 2006.07.09) sets out, a harmonic mitigating device for connection between a power distribution system and the load uses a magnetic shunt to control the levels of magnetic coupling between different elements of the device while providing all windings on a single core. a magnetic core having three core legs, each corresponding with one of three phases of the power distribution system. At least one magnetic shunt, extending across and magnetically coupled to the core legs at an intermediate part thereof, such that first and second parts of the core on either side of the shunt. For each phase, a first reactive element comprising a line winding having a first end for connection to a first line and a second end, the line winding being disposed on the first part of the core. For each phase, a second reactive element comprises a first cross-link winding, disposed on the second part and having a first end connected to the second end of the line winding and a second end connected to a capacitor, the capacitor having a second end connected to a different phase or to a capacitor associated with a different phase. The line winding and associated cross-link winding for each phase are magnetically coupled by the magnetic shunt.

The disadvantages of the described technical solution include the inability to work with decreasing in the active load of more than 50% of the nominal value, which can lead to increasing of THD to a level exceeding the optimum allowable, in particular, more than 5%.

Also in the prior art (patent U.S. Pat. No. 9,356,503 filed 2016 May 31, H02J 1/02) sets out a a harmonic mitigating device that can include at least the following components: an active filter; a passive filter; c) at least one control switch, where the at least one control switch operationally controls whether the at least one active filter or the at least one passive filter can be utilized by the harmonic mitigation device to mitigate at least one harmonic current flow, and where the at least one control switch operationally switches the harmonic mitigation device from utilizing the at least one active filter to utilizing the at least one passive filter when a current load exceeds a predetermined percent of a full current load.

The disadvantages of the described invention include a relatively high coefficient of THD with a load value less than 50% of the nominal, in particular, the THD could be more than 5%. Also, described invention is able to operate on a single frequency power network of 50 or 60 Hz, which reduces its functionality. The claimed technical solution aims solving a technical problems, of prior art.

SUMMARY OF THE INVENTION

A technical result, achieved from implementation of the claimed invention consists in expanding the functionality of the device for suppressing harmonics of the VFD, by increasing the range of load changes, as well as providing the possibility of working from the frequency of the supply network 50/60 Hz.

Thus, even at 40% load the 515-kva VFD capable of keeping the THD level at less than 5%, while TDD shows around 3.33%. When reaching 70% load, the VFD already offers less than 4% THD.

A device for suppressing harmonic distortions at the output of variable frequency drive (VFD) comprising at least one passive harmonic filter (PHF) which has a set of storage elements and a set of inductive elements. Wherein said inductive elements are mounted on a magnetic core. Said PHF comprising at least two lines connected in parallel (parallel lines) with similar sets of elements.

Each of the parallel lines is connected in series with a separate adjustable rectifier of the VFD. At least one of the parallel lines is active, depends on operating mode. Each of the parallel lines comprising a set of storage elements with adjustable contactor, for connection/disconnection at least one of said parallel lines depends on operating mode.

At least a part of the each set of storage elements is active, wherein volume of said sets of storage elements is changeable depending on frequency of input current supply.

The set of inductive elements of said PHF comprising at least two sets of the inductive elements mounted on separate cores and connected through an input. One of the parallel lines is active when the measured active power is less than 50% of its nominal value (half power mode HPM).

The cores of each set of inductive elements contains an upper and a bottom yoke with vertical rods mounted between them. Said vertical rods comprise visible gaps in a cross section, wherein said visible gaps are filled within non-magnetic material. Said visible gaps increased the quality factor of an oscillating circuit, and reducing a dispersion flux.

The cores of each set of the inductive elements are fixed around the perimeter in a frame. Said frame designed with guide elements in a base bottom for improving reparability of the device.

The inductive elements of said PHF are installed in a separate compaitment of the VFD. Said inductive elements are cooled via forced air cooling.

The PHF is configured to change the operating mode between at least two operating modes depending on measured active power and current supply frequency value, wherein at least one of changed operating modes is the half power mode (HPM). During process of changing of the operating modes the control system of the VFD provides a step-disconnection/connection of adjustable contactors of sets of storage elements.

Said PHF has an operating mode, when the active power is upper than 50% of nominal value (full power mode FPM). In this mode the control system provides precharging of sets of storage elements during its step-connection to the at least one of parallel lines.

Also said PHF has an operating mode, when the active power is less than 30% of nominal value (power generating mode PGM).

Also said PHF has an operating mode wherein the set of storage elements is performed to change a storing volume depending on frequency of input current supply (frequency change mode FCM) via connecting an additional capacitor to the set of storage elements by means of the additional adjustable contactor for switching VFD input current supply frequency from 60 Hz to 50 Hz.

BRIEF DESCRIPTION OF THE DRAWINGS

An essence of the claimed invention is explained, but is not limited to the following drawings:

FIG. 1—illustrates variable frequency drive (VFD) with a harmonic suppression device named as passive harmonic filter (PHF);

FIG. 2—illustrates a circuit diagram of the VFD;

FIG. 3—illustrates a circuit diagram of the VFD in a full power operating mode (FPM);

FIG. 4—illustrates a circuit diagram of the VFD in a Half Power Operation mode (HPM);

FIG. 5—illustrates a circuit diagram of the VFD in a frequency change mode (FCM);

FIG. 6—illustrates an inductive element of PHF;

FIG. 7—illustrates a graph of the dependence of the THD in to load;

FIG. 8—illustrates THD level in the output of the VFD.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention implemented in variable frequency/speed drives (VFD/VSD) for various power receivers of non-linear load. In order to reduce the negative impact of consumers on the power supply network, for example in variable frequency drive (VFD) 1 (FIG. 1) of a submersible motor (not shown in the images). In particular any other frequency-controlled electric motor can be used as a load. Housing 2 of the VFD is divided into compartments for electrical equipment installation and contains a separate space formed as single compartment or few compartments for harmonic suppression device elements.

The device 3 for suppressing harmonic distortions is installed on output of the VFD 1. The device 3 contains at least one passive harmonic filter (PHF) (FIG. 2). Said passive filter configured to suppress higher harmonics in particular the 5th and 7th, as well as other groups of harmonics, depending on the user settings. PHF comprises storage elements 4 installed in separate compartment 5 of the VFD which can be represented as capacitor sets.

Said PHF contains a set of inductive elements 7 connected to the sets of storage elements 4; 4,1. Wherein said inductive elements are mounted on a magnetic core 6, designed as reactor choke.

In three-phase (A;B;C) implementation, the PHF contains at least two sets of inductive elements 7 (chokes), installed on separate bases designed as magnetic cores and connected through an input.

Each of phase, contains a set of inductive elements 7 which includes input, output and resonant coils connected in pairs.

Said inductive elements 7 connected to the sets of storage elements 4; 4,1. The elements of the PHF, in particular chokes, are installed in a separate compartment 8 of the VFD. Said compartment 8 is equipped with forced air cooling, implemented by at least one fan 9 of the VFD cooling system.

The PHF comprising at least two lines connected in parallel (parallel lines) 10, 11 (FIG. 2-5), each of the parallel lines 10, 11 is connected in series with a separate adjustable rectifier 12,13 of the VFD for example thyristors of the VFD 1. The present invention also allowed to use different types of adjustable rectifier in which some of the switches are controllable (thyristors) and some of them are uncontrollable (diodes), (diode-thyristor).

Said PHF is configured to work with at least one active line 10 or 11 when the measured active power is less than 50% of its nominal value. This technical solution allows to keep the value of the coefficient of nonlinear distortion (THD) within 5% with a load drop of more than 50% of the nominal value as shown in the graph (FIG. 8).

According to the present invention, an input current is divided between the two parallel lines 10,11 of the harmonic suppression device (FIG. 3). In this case the obtained value of the rated active current of the rectifier is equally to Inom/2.

During operation of the VFD, the current of active load can verify in range 1-100% of the nominal value. During the current decreasing of the active load to 50%, as a result, Inom=Inom/4, which leads to increasing of THD (TDD) coefficient.

During the operation of the VFD, the controller 14 of the control system takes signals from the sensors 15, 16.

Measured current signals Iap allow to perform an on-line monitoring the value of the active power.

At that time, VFD control system determines instantaneous values of active power according to the values of supply voltage and the efficiency of the VFD. VFD control system compares determined values of instantaneous active power with nominal value.

Said VFD control system providing at least two operating modes of the PHF. There are few the most common operating modes of PHF according to the present invention:

-   -   half power mode (HPM)     -   full power mode (FPM)     -   power generating mode (PGM)     -   frequency change mode (FCM)

Half power mode (HPM) is used during the period when control system determines decreasing of the active power values less than permissible value, in particular, more than 50% of the nominal active power value. According to the present invention the permissible value could be set in an interval of acceptable values, for example 45-55% of nominal value.

After determining decreasing of the active power less than permissible value, the controller 14 provides a signal to disconnect one of the parallel lines of the PHF, via switching off the adjustable rectifier 13 of the VFD and contactor 17 of the set of storage elements 4.

Also could be realized another possible implementation of HPM with a programmable dual-mode contactor installed in the input of the PHF, which disconnect one of at least two parallel lines.

After switching off one of at least two parallel lines 10, the current flows through the branch 11 (FIG. 4), with the opened adjustable rectifier 13 (ON). In this moment the value of active power is reaching up close to the nominal value with a minimizing of THD coefficient while the VFD is switched to the “Half power mode”.

During the HPM the control system continues to monitor the current values of active power of the VFD.

When the value of active power increased above setting of values, controller 14 gives a signal “ON” to switch on the adjustable rectifier 13 for inactive line 10 of at least two parallel lines 10, 11 for continuing operation of the VFD in the “Full power mode” (FPM). Also the adjustable contactor 17 is back to close for connecting the set of storage elements 4.

Also, the present invention provides “Power generating mode” (PGM). During the PGM the DC link 18 (FIG. 2-5) of the VFD is pre-charging and providing an active current power to at least 30% of the VFD nominal rate power. In this mode, the sets of storage elements 4, 4.1 of at least one of parallel lines are switched off from the circuit (FIG. 5), by switching off the adjustable contactor 17 until the VFD reaches the setpoint of active power.

After switching on the pulse-width modulation (PWM) and reaching the specified VFD operating parameters, the storage elements 4, 4.1 are connecting to the circuit into both of the parallel lines 10, 11 and the VFD switches on the Full Power or Half Power mode, depending on the measured current value of active power.

During the process of changing of the operating modes the control system of the VFD provides a step-disconnection/connection of adjustable contactors of sets of storage elements. Also said control system provides precharging of sets of storage elements during its step-connection to the at least one of parallel lines. These features allow protecting DC link 18 of VFD from high voltage sags or voltage surges during transient conditions.

Each of the lines of the PHF contains the set of storage elements 4 with the adjustable contactor 17 (FIG. 5), which configured to use a part of volume of storage elements 4.1 depending on frequency of the supply current network.

In particular, the transfer from supply current network frequency of 60 Hz to frequency of 50 Hz is carried out via connecting a part of the storage elements 4.1 to each of the parallel lines 10, 11 by means of adjustable contactor 17.1.

This solution leads to to unification of the VFD, providing the reduction of THD level in the output to 5% or lower, regardless of the frequency of the current supply network (50/60 Hz). FIGS. 7; 8 are showing the graphs of the characteristics of the present technical solution and the method of its operation according to the results of tests and computer simulation of the VFD.

FIG. 7 shows the transfer between Full and Half Power modes, where THD is less than 5%. FIG. 8 shows the result of the mathematical modeling of the VFD operating, which shows reduce rate in the main harmonic groups of the 5th, 7th to the THD value of 3.34%.

The design of the PHF inductive element (FIG. 6) provides ease of installation and maintenance also increases maintainability.

According to the present invention at least two sets of inductive elements 7 are fixed around the perimeter in a frame 19 with guide elements 20 in a base bottom. The magnetic cores 6 of each set of inductive elements 7 contains an upper 21 and a bottom 22 yoke with vertical rods 23 mounted between them. The vertical rods 23 are made with visible gaps 24 in cross section. The visible gaps 24 are filled with non-magnetic material, such as fiberglass or similar. Said visible gaps 24 increased the quality factor of an oscillating circuit, and reducing a dispersion flux.

The present invention can be used for example in different types of six and twelve pulse VFD.

The implementation of the described technical solution helps to solve the present technical problem and expanding the possibility of operating the VFD/VSD with different frequencies of the supply voltage, and also allows to maintain the THD/TDD values within 5% regardless of active power fluctuations. 

What is claimed is:
 1. A device for suppressing harmonic distortions at the output of variable frequency drive (VFD) comprising: at least one passive harmonic filter (PHF) comprising: a set of storage elements and a set of inductive elements, wherein said set of inductive elements are mounted on a magnetic core, said PHF comprising at least two lines connected in parallel (parallel lines) wherein each of the parallel lines is connected in series with a separate adjustable rectifier of the VFD; wherein at least one of the parallel lines is active; wherein each of the parallel lines comprising a set of storage elements with adjustable contactor, wherein at least a part of each set of storage elements is active; wherein a volume of said sets of storage elements is changeable depending on frequency of input current supply; wherein the set of inductive elements of said PHF comprising at least two sets of said inductive elements mounted on separate cores and connected through an input.
 2. The device of claim 1 wherein one of the parallel lines is active when the measured active power is less than 50% of its nominal value.
 3. The device of claim 1 wherein the cores of each set of inductive elements contains an upper and a bottom yoke with vertical rods mounted between them, wherein said vertical rods comprise visible gaps in a cross section, wherein said visible gaps are filled within non-magnetic material.
 4. The device of claim 1 wherein the cores of each set of the inductive elements are fixed around the perimeter in a frame, wherein said frame designed with guide elements in a base bottom.
 5. The device of claim 1 wherein the inductive elements of said PHF are installed in a separate compartment of the VFD, wherein said inductive elements are cooled via forced air cooling.
 6. A method for suppressing harmonic distortions at the output of frequency drive comprising: installing at least one passive harmonic filter (PHF) at the output of a variable frequency drive (VFD), wherein at least one passive harmonic filter (PHF) comprising: a set of storage elements and a set of inductive elements, wherein the set of inductive elements are mounted on at least two separate magnetic cores, wherein said PHF comprising at least two lines connected in parallel (parallel lines) and, each of the parallel lines is connected in series with a separate adjustable rectifier of the VFD; providing at least two operating modes of said PHF; providing an operating mode, when active power is reduced more than 50% of nominal value (half power mode HPM); wherein the HPM comprising: disconnecting one of at least two parallel lines of said PHF via switching off the adjustable rectifier of the VFD, and opening an adjustable contactor of the set of storage elements of said disconnected line of the PHF; configuring the PHF to change the operating mode between at least two operating modes depending on measured active power and current supply frequency value; wherein providing a step-disconnection/connection of adjustable contactors of sets of storage elements during changing of operation modes.
 7. The method according to claim 6, characterized with providing an operating mode, when the active power is upper than 50% of nominal value (full power mode FPM), wherein providing precharging of sets of storage elements during its step-connection to the at least one of parallel lines.
 8. The method according to claim 6, characterized with providing an operating mode, when the active power is less than 30% of nominal value (power generating mode PGM),
 9. The method according to claim 6, characterized with providing an operating mode wherein the set of storage elements is performing to change the storing volume depending on frequency of input current supply (frequency change mode FCM) via connecting an additional capacitor to the set of storage elements via additional adjustable contactor for switching VFD input current supply frequency from 60 Hz to 50 Hz.
 10. The method of claim 6 wherein the PGM comprising: pre-charging a DC link of the VFD, wherein the set of storage elements is switching off from the circuit, via switching off the adjustable contactors till reaching setpoint of the active power of the VFD. 